Re: [patch 15/38] ptp: ptp_vmclock: Replace get_cycles() usage

Arnd Bergmann
2026-04-13T19:31:49Z

On Mon, Apr 13, 2026, at 17:33, David Woodhouse wrote:
> On Fri, 2026-04-10 at 14:19 +0200, Thomas Gleixner wrote:
>
> ... depend on TSC_RELIABLE¹, since if the guest doesn't believe that it
> is, then the guest shouldn't be trying to use it as the basis for
> precise timing.
>
> ¹ (Or... one of the other zoo of TSC flags for the gradually reducing
> brokenness over the years...)

It looks like this is sufficiently handled in the caller:

static int vmclock_get_crosststamp(struct vmclock_state *st,
struct ptp_system_timestamp *sts,
struct system_counterval_t *system_counter,
struct timespec64 *tspec)
{
....
#ifdef CONFIG_X86
/*
* We'd expect the hypervisor to know this and to report the clock
* status as VMCLOCK_STATUS_UNRELIABLE. But be paranoid.
*/
if (check_tsc_unstable())
return -EINVAL;
#endif

With 486 and ELAN out of the way, Winchip6 seems to be the only
one without X86_FEATURE_TSC, so I think the next logical step would
be to turn off Winchip6 as well and remove all X86_FEATURE_TSC
and CONFIG_X86_TSC checks.

Arnd